The invention relates to the field of transmitters for radio frequency communication systems, and particularly relates to transmitters including constant envelope modulation systems.
As wireless communication systems have become increasingly popular, a demand has developed for less expensive yet spectrally clean radio frequency (RF) transmitters having constant envelope modulation systems. High quality RF transmitters typically include relatively expensive components. For example, certain bandpass filters, such as surface acoustic wave (SAW) filters provide excellent performance yet are relatively expensive. Many applications further require transmitters that exhibit low power consumption. It is also desirable that transmitters be suitable for use with any of a plurality of standards for modulation, e.g., global system for mobile communication (GSM) or digital cellular system (DCS).
Constant envelope modulation systems including translation loop modulators are known to provide circuits having relatively less expensive filtering requirements. Translation loop modulators generally include a feedback loop in communication with the output oscillator that is coupled to a transmission antenna. The feedback loop permits the circuit itself to provide bandpass filtering since the output signal may be locked to a given center frequency.
A conventional translation loop modulation system is shown in FIG. 1. The system includes quadrature modulation circuitry 12, phase comparitor circuitry 14, a voltage controlled oscillator (VCO) 16 coupled to an output antenna (not shown), a feedback coupler 17, and a feedback path 18. Input signals representative of the information to be modulated and transmitted may be applied to the I and Q channels of the quadrature modulator. The input signals may be modulated to adjust the phase or angle of a reference signal. This phase information is converted to a voltage signal by the phase comparitor circuitry 14, and the voltage signal is then converted to a frequency signal by the VCO 16. The feedback path 18 provides a phase locked loop to lock the VCO 16 to a given center frequency.
It is conventionally known that transmitter circuits should be designed to reduce the possibility of spurious signals (e.g., harmonics as well as foreign signals) being introduced into the system. In certain situations, the origin of some spurious signals may be extremely difficult to predict, and may be nearly impossible to simulate. To address this problem, it is conventionally believed that transmitter circuits of the type shown in FIG. 1 should be designed to be flexible so that the frequency plan may be adjusted to remove any noise from the band of interest.
For example, in certain situations, a circuit may be most easily corrected by adjusting the frequency of either the voltage controlled oscillator 20 in the phase comparitor circuitry, or the voltage controlled oscillator 22 in the feedback path. Employing two separate oscillators facilitates adjustment for reducing noise since either may be adjusted independent of the other. Moreover, the frequencies may be chosen so as to not be harmonically related, which minimizes the chance of harmonic spurious signals being produced by the oscillators.
Unfortunately, however, some oscillators are rather expensive. For example, certain oscillator circuits that are formed of synthesizers produce very stable output signals, but are relatively expensive. It is also desirable that the use of relatively expensive filters be avoided.
There is a need, therefore, for inexpensive yet efficient constant envelope modulation systems. There is further a need for a translation loop modulator that is spectrally efficient yet economical to produce.